Scientists do their best work when faced with contradictory results. If you always get just one result, then what you are investigating isn’t very interesting. But if sometimes you see one thing and sometimes you see another, then that’s Nature’s way of telling you that you are on the verge of learning something truly neat. And that’s what happens to Mary, Peter, and Daniel today as they look for the silver lining.
The atmosphere in Peter’s living room was just perfect for the Secret Science Society’s annual “Mad Science Movie Marathon”. While Mary, Peter, and Daniel indulged in huge bowls of popcorn, plates of caramel apples, and glasses of swamp juice (lemon soda with food coloring and raisins), classic monster movies from the 1950s ran on the DVD player and a fierce storm raged outside. They had laughed at The Mummy’s bad hieroglyphics, howled along with The Wolfman, and shivered as Frankenstein brought his creation to life with the lightning on the screen being echoed by real thunder from the storm outside. Naturally, just as the villagers gathered up their pitchforks to explain the homeowner association rules to poor, mad Victor, a bright flash of light and an ear-shattering crack told of a near-miss and the television and lights and all other power went off in the house.
“Don’t worry,” Peter said. “I know where the emergency flashlights are.”
“Rats!” said Daniel. “It was just getting good!”
“I wonder how long it will take to get the power back,” Mary mused. “And what will we do while we wait?”
“I’ve got a better question,” Daniel said. “Why is it dark?”
“Huh?” said Peter as he came back into the room with three flashlights.
“Think about it,” Daniel said. “When you look at a cloud on a sunny day, the cloud is white. Sometimes it even seems brighter than the sky around it. So why is it dark under a rain cloud? Aren’t they all the same thing?”
“I hadn’t thought about it,” Mary replied. “But you are right. Rain clouds are dark but regular clouds aren’t. I wonder why?”
“Well, it is too wet outside to go ask Mr. Medes,” Peter said. “Do you think my mom might know?”
“Might know what?” Peter’s mother asked as she came into the room with more flashlights. “I thought you might need these but I see you’ve got things well in hand!”
“Daniel asked something that we don’t know the answer to,” Mary said. “Why is it dark when it rains if clouds are white?”
“Well, there’s no shame in not knowing something. The only shame is if you don’t try to find out what the answer is,” Peter’s mother replied. “And it turns out that the answer to your question happens to apply to my work. So, yes, I know the answer.”
“What is it?” Daniel asked.
“Well, would you rather I told you or would you prefer to do an experiment?”
“Experiment! Experiment!” the three young scientists chorused.
“OK. Peter, go get that bag of marbles from your room,” his mother directed. “And I’ll go get some clear plastic bags from the kitchen. We’ve already got flashlights, so we’re all set.”
Peter quickly went to his bedroom and grabbed the bag of marbles. As he came back into the den, his mother returned with four plastic bags. Taking the marbles from Peter, she filled each bag with marbles before sealing it and handing it to one of the scientists.
“OK,” she said as she filled her bag with marbles. “This would work better if the marbles were clear instead of having that swirl of color in the middle, but it is close enough for our purposes. What I want you to do is shine your flashlight through the bag of marbles cross-wise so that the light goes through the ‘thin way’. What happens?’
“I can see the light but it is a bit fuzzy,” said Daniel.
“And the edge of some of the marbles gets bright,” added Mary.
“Good,” said Peter’s mother. “Now, what I want you to do is shine the light through the bag of marbles the long way. But, before you do, tell me – what will you see?”
“Probably the same thing we just saw,” said Peter. “The light will be fuzzy and there will be some bright edges.”
“I don’t know,” said Daniel. “Maybe having more marbles means that the light won’t make it through somehow.”
“Or maybe we’ll just see bright edges,” added Mary.
“Well, there’s only one way to find out!” Peter’s mother said.
What do you think will happen? Try the experiment yourself!
The three turned their baggies longwise and looked at the flashlight shining through. But instead of a bright light, they only saw a dull, fuzzy beam. The marbles had dimmed the flashlight beam just as clouds dulled sunbeams.
“Wow!” exclaimed Peter. “The light got a lot darker.”
“And most of the bright edges are gone!” said Mary.
“But why?” asked Daniel.
“The reason for this is the same reason that the bottom of the ocean is dark and that radio waves don’t travel very far in a nebula,” Peter’s mother said. “It is a type of physics known as optics. When the light from the sun or from a flashlight beam hits an object, three things happen: reflection, refraction, and absorption.”
“Reflection like a mirror?” Mary asked.
“Exactly! You may have noticed that you can see your face in a very still pond; that’s because some of the light that hit the top of the water was reflected back at you,” Peter’s mother explained. “The same thing happens with our marbles and with the raindrops that make up a cloud. Some of the light gets reflected back off of every raindrop. As you get deeper into the cloud or the cloud gets thicker, less and less light makes it through.”
“Oh, so that’s why rainclouds clouds are dark! They are thicker than other clouds!” Peter said.
“No, that’s only part of the explanation,” his mother replied. “There’s also refraction; that’s what happens when the light gets bent by the raindrop. Instead of traveling through and continuing in a straight line like a toothpick in an olive, the raindrop makes the path of the light shift a little so it looks more like a broken toothpick in an olive. And because the angle of the break is different for each color of light, when the angle is just right, you can get -“
“A rainbow!” Daniel said. “Is the bent light what made the edges of the marbles seem bright?”
“That’s exactly right,” Peter’s mother said. “Taken together, we sometimes refer to reflection and refraction as scattering. But reflection and refraction are only part of the reason that rain clouds are dark. The third reason is – “
“Absorption!” Mary said. “Is that like when a sponge absorbs water?”
“Not quite,” Peter’s mother said. “With a sponge, you can always get the water back out by squeezing it. But when light gets absorbed by a raindrop, it gets changed into heat. That added energy might make the raindrop warm up a very little bit or it might be re-radiated as infrared light. And since we can’t see in infrared, that makes it dark in the center of a rain cloud and under one, too.”
“But what does that have to do with the ocean bottom?” Peter asked.
“You can think of the ocean as a whole bunch of raindrops jammed together,” his mother replied. “As the light goes through the ocean, some of it gets absorbed. Interestingly, the depth that the light makes it down to depends on the wavelength of the light. Colors like red have very long wavelengths and make it deeper into the ocean than colors like blue. In addition, water like to scatter the shorter wavelength colors like blue; that’s why the ocean looks blue – more of that color gets reflected to your eyes. Taken all together, the amount of light that you can see in the ocean drops by 90% for every 75 meters. So if the ocean was as deep as a skyscraper is high, the bottom floor would get only 10% as much light as the top one would.”
“Cool!” Daniel said. “But what does that have to do with your work?”
“I’m a planetologist,” she replied. “That means that sometimes I look at planets before they are born, when they are just big clouds of gas and dust called nebulae. The gas and dust in a nebula will scatter and absorb light just like the water in the ocean or the raindrops in a cloud. And by measuring how the light from stars behind the nebula is scattered and absorbed, we can estimate the thickness of the cloud and even learn what it is made of. We’ve found water, ammonia, formaldehyde, and even amino acids in nebulae across the galaxy. There are even some scientists who think that life on Earth started thanks to those amino acids.”
Just then, the power came back on.
“Well, it looks as if your creation has come back to life,” Peter’s mother said. “So I’ll just leave you three to your movies.”
“Thanks mom!” Peter said, his fingers already on the remote, ready to start the movie again as the three sat back down absorbed once more in the morality tale on the silver screen.